After years of delays, four former guards from the security firm Blackwater Worldwide are facing trial in the killings of 14 Iraqi civilians and the wounding of 18 others in bloodshed that inflamed anti-American sentiment around the globe.
Whether the shootings were self-defense or an unprovoked attack, the carnage of 16 September 2007 was seen by critics of the George W Bush administration as an illustration of a war gone horribly wrong.
A trial in the nearly seven-year-old case is scheduled to begin with jury selection on Wednesday, barring last-minute legal developments. Prosecutors plan to call dozens of Iraqis to testify in what the Justice Department says is likely to be the largest group of foreign witnesses ever to travel to the US to participate in a criminal trial.
The violence at the Nisoor Square traffic circle in downtown Baghdad was the darkest episode of contractor violence during the war in Iraq, becoming one more diplomatic disaster in a war that had many. Iraqi officials, who wanted the guards tried in a local court, were outraged.
In the trial, defense lawyers will focus on the guards' state of mind in a city that was a battleground.